The invention relates to a process for the manufacture of opaque quartz glass by forming an initial body from synthetic SiO2 granules and heating said body at a vitrification temperature to obtain a preform of opaque quartz glass.
Furthermore, the invention relates to a synthetic granulate suitable for the process, said granulate being prepared from at least partially porous agglomerates of SiO2 primary particles.
Opaque quartz glass is primarily utilized in heat-technology applications where good thermal insulation and high temperature stability are of importance. Increasingly higher demands are made in these fields regarding the purity of such quartz glass bodies. The following examples of applications in the semiconductor industry may be mentioned: reactors, diffusion tubes, heat shields, bells or flanges. Primarily, opacity in the IR spectrum is required in these applications. Opacity means in this context low transmittance (less than one percent) both in the visible (approximately form 350 to 800 nm) and the IR spectra (approximately from 750 to 4,800 nm). In quartz glass of low purity the desired opacity occurs by itself due to the contaminants contained therein. On the other hand, when pure source materials are used, transparent quartz glass is obtained so that the opacity must be created by artificially introduced pores. The subject of the present invention is the production of opaque quartz glass from pure source materials.
A process of this kind for the production of opaque quartz glass from pure source materials is described in EP A1 816,297. It is proposed there to create opacity in the quartz glass by making and melting a powder mixture of synthetically produced SiO2 particles having an average particle size of 300 xcexcm and an additive material in form of powdered silicon nitride. Gaseous components such as nitrogen are released during the melting of the powder due to the thermal decomposition of the Si3N4 powder. The gaseous components cause bubbles to be formed in the softened quartz glass thus creating the desired opacity of the body. A graphite mold lined with graphite felt is filled with the powder mixture and heated in an electric furnace in a vacuum at a temperature of 1,800xc2x0 C. During the melting the frontal region of the softening and melting quartz glass advances as a xe2x80x9cmelt frontxe2x80x9d radially outward from the walls of the mold.
Devitrification of the quartz glass can take place due to contaminants, leading to brittleness and reduced resistance to temperature change. Remnants of the additive material can also diminish the quality of the quartz glass in this respect. Pore growth can also take place during vitrification, with large pores growing at the expense of smaller ones. However, large pores contribute little to opacity, lead to a lesser density of the opaque quartz glass and reduce the mechanical strength of the quartz glass body.
A SiO2 granulate of this kind is known from DE A1 44 24 044. It is proposed therein that an aqueous suspension of silicic powder manufactured in a pyrogenic process be treated in a mixing container by rotating agitators. The rotational velocity in a first mixing phase is between 15 and 30 m/s, and 30 m/s or more in a second mixing phase. The solids content of the suspension is at least 75% by weight during the first mixing. A granular mass with an average granule diameter of less than 4 mm results from the first mixing phase. The granular mass is further consolidated by addition of amorphous silicic dust and the granular mass is reduced in size in a second mixing phase by intensive mixing and beating. Water emerges from the surface of the granular mass and is absorbed by addition of more silicic dust in order to prevent lumping of the granulate. The known process provides a pourable SiO2 granulate of high powder density, suitable for applications as filler in dentistry or as catalyst support.
The object of the invention is to provide a process for the manufacture of pure opaque quartz glass with homogenous pore distribution and also with high density, high viscosity and low tendency to devitrify. The object of the invention is also to provide a SiO2 granulate suitable to carrying out the process.
As concerns the process, the object is achieved according to the invention on the basis of the process cited initially in that the employed SiO2 granulate is formed from at least partially porous agglomerates of SiO2 primary particles, has a specific BET surface ranging from 1.5 m2/g to 40 m2/g and an apparent density of at least 0.8 g/cm3.
Vitrification of a starting body formed from at least partially porous agglomerates of SiO2 primary particles with a specific BET surface ranging from 1.5 m2/g to 40 m2/g to an apparent density of at least 0.8 g/cm3, produces opaque pure quartz glass exhibiting homogenous pore distribution and high density, high viscosity and low tendency to devitrify. An article produced from the opaque quartz glass is distinguished by good heat insulation and long service life at high temperatures.
The SiO2 granulate is present in form of at least partially porous agglomerates of SiO2 primary particles. Such primary particles are obtained by for example flame hydrolysis or oxidation of silica compounds, by hydrolysis of organic silica compounds in a so-called sol-gel process or by hydrolysis of inorganic silica compounds in a liquid. Even though such primary particles stand out due to their high purity they are difficult to handle due to their low powder density. Therefore compacting by granulation processes is conventionally used. Agglomerates with greater diameters are formed due to the lumping together of fine primary particles. These agglomerates have a number of open pore channels, forming a correspondingly large volume of pores. The individual granules of the SiO2 granulate used in the process according to the invention are formed from such agglomerates. Due to the large pore volume the granulate is distinguished by a specific BET surface ranging from 1.5 m2/g to 40 m2/g. This surface therefore does not appear as outer surface but predominantly as inner surface in form of pore channels. During vitrification of the starting body the larger part of the pore volume closes due to sintering and collapsing. However, a large number of fine closed pores remains from the pore channels that were previously open. Incident light is diffused by the closed pores, resulting in opacity or low transmittance. The large surface is favorable for the formation of gaseous silicon oxide (SiO) during vitrification which counters the collapse of small pores since gases trapped in closed pores can no longer escape.
Therefore no additive material volatile at vitrification is needed to achieve opacity as is the case in the known process described initially. As a result contaminants which go hand in hand with the use of such additives can be avoided.
According to the invention, the synthetically manufactured SiO2 granulate used is distinguished by a specific surface ranging from 1.5 to 40 m2/g and at the same time by high apparent density. The density of at least 0.8 g/cm3 primarily assures that the starting body can be formed from the granulate, while the opacity of the quartz glass is substantially the result of the large specific surface, as explained above.
The specific surface of the SiO2 granulate is determined according to the BET process (DIN 66132), and the apparent density according to DIN/ISO 787, Part 11.
From the SiO2 granulate the starting body is formed as loose fill or as a thermally or mechanically pre-compacted body.
It has been shown that a particularly suitable SiO2 granulate for use in the process according to the invention has a specific BET surface ranging from 10 m2/g to 30 m2/g. Good results are achieved regarding opacity or low transmittance of the quartz glass, especially in the IR spectrum, with high density and low devitrification tendency. Especially good results regarding pourability and ease of handling of the granulate were achieved at an apparent density ranging from 0.9 g/cm3 to 1.4 g/cm3.
In a preferred variant of the process the SiO2 primary particles have an average particle size ranging from 0.5 xcexcm to 5 xcexcm. Such primary particles are obtained in a so-called xe2x80x9csol-gelxe2x80x9d process by hydrolysis of organic silicon compounds. In an alternative and equally preferred variant the SiO2 primary particles have an average particle size of less than 0.2 xcexcm. Such pyrogenic particles are obtained by flame hydrolysis or oxidation of inorganic silicon compounds. With a view to low devitrification tendency during vitrification the primary particles are preferably amorphous.
In both process variants the primary particles are distinguished by a large free surface. Granulates in the sense of the present invention are formed by agglomeration of a plurality of such primary particles due to physical or chemical binding forces. Known granulating processes are used, especially wet granulation or extrusion of a mass containing the primary particles. Primary particles produced especially according to the sol-gel process are densely compacted when present in the granulate since they are overwhelmingly and also preferably spherical. The free surface is reduced by the contact surface of adjoining primary particles; however, as explained above, closed pores can develop between the individual primary particles during vitrification. Because the primary particles have an average size of less than 5 xcexcm, the resulting pore distribution is correspondingly fine. The average primary particle size is determined as the so-called D50 value according to ASTM C1070.
It has been shown that in a granulate particularly suitable for use in the process according to the invention the individual SiO2 granules have an non-homogenous density distribution and the inner region of lesser density is at least partially enclosed by an outer region of higher density. It is therefore possible to enclose in the inner region gases which then do not escape or only partially escape during vitrification and thus contribute to development of pores and opacity (low transmittance) of the quartz glass.
In a preferred implementation of the process use is made of granulates in which the inner region encloses a hollow space. The hollow space is enclosed at least partially by the outer region of higher density.
The outer region of porous granulates is advantageously pre-compacted by thermal treatment comprising sintering at a temperature ranging from 800xc2x0 C. to 1,350xc2x0 C. The thermal treatment gives the outer region a density that is higher than that of the porous or hollow inner region so that pores and pore channels in the outer region preferably shrink and close. In order to achieve this the thermal treatment is stopped or interrupted before the equalization of the initially established temperature gradient between the outer and inner region. This can be realized in a simple way by for example continuously moving the granulate through a heating zone.
A thermal treatment comprising heating in an atmosphere containing chlorine has been shown to be useful. Treatment in a chlorine-containing atmosphere removes contaminants which form chlorine compounds volatile at treatment temperatures, and OH compounds. This improves the purity of the opaque quartz glass, increases viscosity and further reduces devitrification tendency. The chlorine-containing atmosphere contains chlorine and/or a chlorine compound. In a quartz glass pure in the sense of the present invention the total contamination by Li, Na, K, Mg, Ca, Fe, Cu, Cr, Mn, Ti and Zr is less than 250 ppb by weight. Dopants are not considered contaminants in this sense.
In a preferred method the thermal treatment comprises heating of the porous agglomerates at a temperature ranging from 1,000xc2x0 C. to 1,300xc2x0 C. in a nitrogen-containing atmosphere and under presence of carbon. By this method, hereinafter xe2x80x9ccarbo-thermal nitriding,xe2x80x9d granules are obtained whose entire free surface is enriched by nitrogen. Nitrogen fixation is facilitated by the presence of carbon, but the latter volatilizes. It has been shown that the viscosity of the quartz glass increases due to nitrogen content. High viscosity can also be achieved with a granulate composed of SiO2 granules doped with aluminum at a level ranging from 5 to 20 ppm by weight. Aluminum doping is advantageously effected by finely distributed nano-scale Al2O3 particles, assuring a homogenous distribution of the dopant. Particularly well suited therefor are pyrogenically produced Al2O3 particles because of their high specific surface.
In a first preferred embodiment the granulate is composed of rounded SiO2 granules. Such a granulate is distinguished by good pourability, high apparent density and low shrinkage during vitrification and is obtained by for example the wet granulation process.
In a second equally preferred embodiment the granulate is an extrudate. Such granulate can be manufactured inexpensively by extrusion. It is distinguished by elongated SiO2 granules which may also contain elongated hollow spaces.
The granulate according to the invention is particularly suited for the production of an opaque quartz glass article such as a heat shield, a reaction container and a muffle.
Such an article is distinguished in that it has an opaque quartz glass region formed from a synthetic SiO2 granulate with a specific BET surface ranging from 0.5 m2/g to 40 m2/g and, apparent density of at least 0.8 g/cm3 and formed from at least partially porous primary particles.
The opaque region is obtained by vitrifying the appropriate region of the fill. It is distinguished by high opacity in the IR spectrum, achieved substantially by forming the region from a SiO2 granulate of partially porous agglomerates of SiO2 primary particles, and having a specific BET surface ranging from 1.5 m2/g to 40 m2/g. Vitrification of such SiO2 granulate results in an opaque quartz glass with a homogenous particle distribution and high density. This allows a particularly high thermal insulation effect to be attained when the article is used as intended. During granulation the fine primary particles lump together forming agglomerates with a larger diameter. These agglomerates have a plurality of open pore channels, forming a correspondingly large pore volume. The individual granules of the employed SiO2 granulate are formed from such agglomerates. During vitrification the larger part of the pore volume closes due to sintering and collapsing. However, a large number of fine closed pores remains from the pore channels that were previously open and IR radiation is diffused by them, resulting in opacity in the IR spectrum.